In dimension gauging such as capacitive distance gauging, or inductive resistivity gauging, it is common to have the output of the capacitor or inductive sensor inversely representing the parameter of interest. For example, in the case of capacitive distance gauging, the output will vary inversely with the dimension forming the distance between the capacitive elements. For typical use in providing an output indication of the parameter of interest, the inversely varying sensor output is linearized or inverted before it is applied to a visual readout or other processing systems.
Systems are known which may be used to provide this inversion function, in particular those represented by our U.S. Pat. Nos. 3,805,150, 3,775,679 and 3,986,109, 3,990,005. While satisfactory, these systems involve additional electronic components to provide the inversion function.
In the case of resistivity measurement of an element, it is common to have an inductive eddy current sensor providing an output indication directly representative of conductance and at the same time inversely representative of resistance. It is again convenient to provide the ultimate output directly varying with the resistance gauged, and even more preferably, the resistivity necessitating a further mathematical manipulation to eliminate the dimensional variation.
In the present invention, this function is achieved by digitizing the sensor output in a dual slope integrator. The up integration is responsive to a first signal with which the resulting digitized output varies directly and the down integration is in response to a further signal, typically the sensor output, with which the digitized output varies inversely. As a result, the digitized output is made to vary directly with the ultimately desired parameter.
Dual slope integrators, as they are known in the art, generate cumulative offsets and other errors which can greatly reduce the accuracy. Therefore, a compensation feedback loop is often provided around the dual slope integrator with means for storing an analog representation of the error. This representation is provided as a compensating signal to the dual slope integrator. The thus compensated integrator, however, is still subject to other system errors. Accordingly, in a further aspect of the invention, a second closed loop is provided around the entire system from the transducer to the output of the dual slope integrator. This cooperates with the first loop to reduce entire system errors. The compensating systems are preferably activated in two distinct operating cycles. In the first, the dual slope integrator gauging function alternates with a compensation interval provided by the first closed integrator loop around the dual slope integrator. This cycle provides actual gauge operation for the detection of a desired dimension. Alternating with this cycle is a calibration cycle in which the first closed loop is enabled in alternation with the second feedback loop around the entire system. This latter cycle typically occurs during intervals of sensor inactivity between dimension readings.